Ron Unz’s argument  that Hispanics commit no more crime on average than whites has  caused  a stir . Some immigration restrictionists are not pleased . Still, it seems to me that restrictionists (and I count myself one) should welcome any effort, including Mr. Unz’s, to identify and understand the actual effects of mass immigration, even if they turn out to be benign.
Restrictionists, after all, are playing with a handicap, for the merits of immigration policy inevitably turn on statistical judgments as to the characteristics of different populations (Mr. Unz’s article being an example). Not long ago, analyzing these differences posed no particular intellectual problem. Everyone “stereotyped,” most often casually, sometimes pseudo-scientifically, and in a few cases rigorously. Today, by contrast, study of group differences has become disreputable. Many now see it as a moral obligation to assume instead that differences among populations (whatever their nature or source) either do not exist or that appearances to the contrary are illusory. That any rational discussion of immigration, pro or con, takes place at all is itself a small victory for restrictionists.
On the other hand, immigration enthusiasts who welcome  Mr. Unz’s findings should not crow. Nobody who has studied the question thinks it’s easy to tease out of the data the relative crime rates of immigrants and natives. Even if Mr. Unz’s article ultimately proves correct, we still would not know exactly what effect immigration now has on crime. Defenders of mass immigration, however, cannot hang their case on merely the tentative conclusion that immigration probably does not increase crime. Ensuring citizens’ personal safety, after all, is one of the most important duties of the State. (Indeed, that’s pretty much what government is for.) Given the disastrous consequences if Mr. Unz is wrong, immigration proponents have the burden or proving with reasonable certainty that immigration will not make Americans less safe. On immigration, the Precautionary Principle  must govern.
I can already hear worldly libertarian pundits deriding this position. Hasn’t the Precautionary Principle been debunked  as incoherent ? As Cass Sunstein writes in his book Worst-Case Scenarios , the Precautionary Principle condemns both action (“Even if we don’t know whether immigration increases crime, so long as there is at least a possibility that it will, we should restrict immigration”) and inaction (“Even if we don’t know whether restricting immigration will hurt our economy, so long as there is at least a possibility that it will, we should continue mass immigration”). Yeah, yeah I know: the Precautionary Principle, as invoked by the more naive type of environmentalist, is incoherent.
But that’s not the end of the story. Sunstein goes on to acknowledge that more limited forms of the Precautionary Principle are defensible. In particular, the Irreversible Harm Precautionary Principle holds that if an action could cause irreversible harm, then we should restrict it until we can figure out what its consequences will actually be. Or, as economist Kenneth Arrow put it  in his paper introducing the idea, if an action “involves some irreversible transformation of the environment, hence a loss in perpetuity of the benefits of preservation, and if information about the costs and benefits of both alternatives realized in one period results in a change in their expected values for the next,” then the action should be at least partially restricted. In other words, we should be willing to pay a price just to keep our options open. The rationality of this Precautionary Principle is demonstrated every day in the financial markets, whenever investors buy options to hedge their future bets.
Now, it’s hard to imagine a policy more irreversible than immigration. Once people arrive, they tend to stick around, for better or for worse, indefinitely. Meanwhile, the harms that bringing in a new population may cause are enormous. They include not just increased crime, but the whole litany restrictionist fears, from depressed wages and increased inequality to loss of social trust and the decaying of America’s political legitimacy (not to mention the non-monetizable but still very real loss of the culture many simply happen to prefer). We don’t even have the data yet to know whether immigration is making our society more violent. Can’t we just wait to figure it out before we continue current policy? That would seem to be the most sensible way to proceed, regardless of whether Mr. Unz turns out to be right.